In recent years, however, sleep apnea, which causes sleepers to stop breathing dozens of times during the night, has emerged as a serious threat to health and longevity. Research has linked it to high blood pressure, heart disease, stroke, diabetes, irregular heartbeats, depression, and even cancer.

Now a recent study published in the Journal of the American Medical Association has produced compelling evidence that sleep apnea contributes to cognitive decline and dementia, too, apparently by causing hypoxemia — a drop in the level of oxygen in the blood.

"We've suspected that for a long time, but the problem has been linking hypoxemia to cognitive stress," said Dr. W. McDowell Anderson, program director of the USF sleep medicine training program, and medical director of Tampa General's Sleep Disorder Center. "The way they designed this study allowed them to conclude that obstructive sleep apnea and hypoxemia are directly related to cognitive dysfunction."

The study, led by Dr. Kristine Yaffe of the University of California, San Francisco, followed 298 older women who were free of cognitive problems. Slightly more than one-third of the women had sleep-disordered breathing, including at least 15 episodes each night in which they stopped breathing long enough for the levels of oxygen in their blood to drop.

Over five years, nearly 45 percent of those women developed mild cognitive impairment or dementia compared to only 31 percent of the women without sleep problems. In addition, the researchers were able to determine that the cognitive problems were related to hypoxemia rather than to sleep fragmentation or lack of sleep.

Another small study published in the May 2011 issue of the American Journal of Respiratory and Critical Care Medicine found that 17 patients who were not treated for sleep apnea displayed impairments in cognition and mood associated with brain shrinkage in the left hippocampus and other areas associated with memory creation.

After continuous positive airway pressure treatment, or CPAP, those areas of the brain increased in volume, and the patients displayed improvements in memory, attention and planning. An editorial in the same issue called for earlier diagnosis and more aggressive treatment of sleep-disordered breathing, which is the third-most common respiratory disorder after asthma and chronic obstructive pulmonary disease (COPD). If the drop in blood oxygen caused by sleep apnea can be treated "this would argue in favor of early treatment, even in patients who are not subjectively sleepy," said an editorial in the same issue.

In addition to hypoxemia, sleep deprivation produces plenty of other problems, such as extreme daytime sleepiness, irritability, depression, and an inability to concentrate. People with sleep apnea are also far more susceptible to accidents.

"The most severe outcome of sleep deprivation is death from a car or industrial accident," Dr. Anderson said. "Treatment for sleep apnea decreases accidents, especially auto accidents."

There are two kinds of sleep apnea — obstructive sleep apnea (OSA), which occurs when the tongue and soft palette relax against the back of the throat and block the airway, and central sleep apnea, which results from weak signals from the brain to the diaphragm, which draws air into the lungs when it contracts.

OSA can be treated easily with a small machine that blows a steady stream of air into a mask worn by the sleeper. This CPAP treatment usually produces normal sleep patterns immediately all by itself, but some people can't tolerate the mask, and resort to a device that resembles a football mouth guard, which pushes the lower jaw forward and holds the airway open. Various surgical options are available as well, although they tend to be reserved for cases that resist all other treatments.

Central sleep apnea, which is caused by congestive heart failure, stroke, brain damage, drugs, and other problems that weaken signals from the brain to the diaphragm, may respond to CPAP, but it usually requires bilevel positive airway pressure, or BPAP, which produces higher pressure during inhalation and lower pressure during exhalation. Some BPAP machines even provide breathing assistance when they detect that the sleeper has stopped breathing.

And more treatments are under development, according to Dr. Anderson. One involves a small device similar to a pacemaker that is surgically implanted under the skin, and attached to a wire that runs to the hypoglossal nerve, which controls the tongue. "During the night, it stimulates the nerve, which increases tone in the tongue so it doesn't fall to the back of the throat and block the airway," he said. "The device remains inactive when you're awake."

He hopes that the recent paper linking sleep apnea to cognitive decline and dementia will spur all doctors to screen patients for signs of disordered sleep.

"When I was in medical school, we weren't even taught about sleep apnea," he said. "Now, even though little time is spent on sleep apnea, it is taught, but it hasn't caught on very much considering it's as prevalent as diabetes, and linked to numerous abnormalities. At least a lot more information about it is finally getting out there."

Tom Valeo writes frequently about health matters. He can be reached at [email protected]

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